Traditionally OCT has been used to capture information regarding the interior of an object, or specifically its differential structure based on back scattering, reflectance distribution and refractive index distribution, in a non-destructive manner at high resolution.
One non-destructive tomographic measurement technology used in the medical field, etc., is optical coherence tomography (OCT) (refer to Patent Literature 1). OCT uses light as a measuring probe and therefore provides the benefit of allowing for measurement of reflectance distribution, refractive index distribution, spectroscopic information, polarization information (double refractive index distribution), etc., of the measuring target.
Basic OCT 43 uses the Michelson interferometer whose operating principles are explained using FIG. 8. The light emitted from a light source 44 is parallelized by a collimating lens 45 and then split into reference light and object light by a beam splitter 46. The object light is focused onto a measuring target 48 by an objective lens 47 in the object arm, and the scattered/reflected light returns to the objective lens 47 and beam splitter 46.
On the other hand, the reference light passes through an objective lens 49 in the reference arm and then reflects on a reference mirror 50, travels through the objective lens 49 again, and returns to the beam splitter 46. The object light and reference light thus returning to the beam splitter 46 enter a condensing lens 51 and get focused onto an optical detector 52 (photodiode, etc.).
For the OCT light source 44, a light source capable of generating light of low temporal coherence (interference between lights emitted by the light source at different times is kept to a minimum) is used. With the Michelson interferometer using a light source of low temporal coherence light, interference signals manifest only when the distance from the reference arm is roughly equivalent to the distance from the object arm. This means that, when the intensity of interference signal is measured with the optical detector 52 while changing the differential optical path length (τ) between the reference arm and object arm, interference signals at varying differential optical path lengths (interferogram) can be obtained.
The shape of this interferogram represents the reflectance distribution of the measuring target 48 in the depth direction, and a one-dimensional scan of the interferogram in the axial direction reveals the structure of the measuring target 48 in the depth direction. With OCT 43, therefore, the structure of the measuring target 48 in the depth direction can be measured by optical path length scanning.
A two-dimensional scan combining the aforementioned scan in the axial direction (direction A) with a mechanical scan (scan B) in the lateral direction (direction B) gives a two-dimensional section image of the measuring target. A scanner that performs this lateral scan may be configured in such a way that the measuring target is moved directly, or it may be configured in such a way that the target is fixed and the objective lens is shifted, or it may be configured in such a way that both the measuring target and objective lens are fixed and the galvano-mirror near the pupil plane of the objective lens is turned to change its angle, or the like.
Advanced versions of the basic OCT mentioned above include swept source OCT (SS-OCT) where the wavelengths of the light source are scanned to obtain spectral interference signals, and spectral domain OCT where a spectroscope is used to obtain spectral signals. The latter includes Fourier domain OCT (FD-OCT; refer to Patent Literature 2) and PS-OCT (refer to Patent Literature 3).
In swept source OCT, a high-speed wavelength scanning laser is used to change the wavelength of the light source, after which spectral signals and synchronously acquired light source scanning signals are used to rearrange the interference signals, which are then processed to obtain a three-dimensional optical tomographic image. Swept source OCT can also use a monochrometer as the means for changing the wavelength of the light source.
Fourier domain OCT is characterized in that a wavelength spectrum of the reflected light from the measuring target is acquired by a spectrometer, and this spectral intensity distribution is Fourier-converted to retrieve signals in real space (OCT signal space), and therefore Fourier domain OCT does not require scanning in the depth direction, but scanning in the X-axis direction permits measurement of the section structure of the measuring target.
PS-OCT provides an optical coherence tomography device that continuously modulates the polarized state of the linearly polarized beam simultaneously during scan B, to capture the polarization information of the sample (measuring target) and thereby permit measurement of the more detailed structure and anisotropy of refractive index of the sample.
To be more specific, PS-OCT acquires a wavelength spectrum of the reflected light from the measuring target using a spectrometer, just like Fourier domain OCT, but is different in that the incident light and reference light are horizontally and linearly polarized, vertically and linearly polarized, linearly polarized at 45°, and circularly polarized through a ½ wavelength plate, ¼ wavelength plate, etc., respectively, after which the reflected light from the measuring target and the reference light are superimposed with each other and the superimposed light is passed through a ½ wavelength plate, ¼ wavelength plate, etc., to, for example, allow only the horizontally polarized component to enter the spectrometer to cause interference, thereby retrieving and Fourier-converting the component associated with the specific polarized state of object light. This PS-OCT does not require scanning in the depth direction, either.